1. Field of the Invention
The present invention relates to wavelength lockers, and more particularly to a wavelength locker suitable for use with a tunable laser.
2. Description of the Related Art
Laser frequency monitoring and locking is an essential technology for optical communications and other fields. For instance, optimization of a dense wavelength division multiplexing (DWDM) system requires tight control and accurate tuning of each frequency propagated down an optical fiber by a communication laser. In DWDM, each of a plurality of laser signal sources is tuned in frequency to a distinct channel, allowing a plurality of signals to be simultaneously transmitted down a single optical fiber. In this way, large volumes of information can be transmitted through a single optical fiber. The communication channels are defined on a grid with equal frequency spacing in a band near 194 THz (the ITU grid).
Each laser must be stabilized, or xe2x80x9clocked,xe2x80x9d to a wavelength locker so as to ensure it remains frequency-tuned to the proper communications channel, regardless of environmental or systematic disturbances. A wavelength locker provides a stable and calibrated reference for measuring the wavelength deviation of a laser output from a desired wavelength, such as an ITU communications channel. The signal from the wavelength locker is used to tune the laser wavelength back to the desired communication channel. Mistuning is highly detrimental to the performance of a DWDM system since many DWDM components exhibit wavelength-dependent losses. Wavelength lockers are critical to telecommunication systems because they allow for more closely spaced channels, thus increasing the information bandwidth of a DWDM system.
For DWDM systems where the communications channels are spaced equally apart in optical frequency, it is common to use an interferometric optical element, such as a Fabry-Perot (FP) etalon, as the reference element of a wavelength locker, as shown in FIG. 1. An FP etalon is a simple interferometric device composed of two partially-reflecting mirrors that are substantially parallel and separated by a gap. The transmission of light through an FP etalon is periodic in wavelength and is expressed as an Airy function (Optics, 2nd Ed., Hecht, p.367). The separation in frequency between the periodic peaks of the transmission response is called the Free Spectral Range (FSR), and it depends on the optical path length of the gap between the etalon mirrors. The frequency dependence of the transmission response of an etalon can be used as a discriminator for locking the laser optical frequency. A common strategy for wavelength locking is to match the FSR of the etalon to the frequency spacing of the ITU grid. The FP etalon acts as a calibrated reference to indicate where the ITU channels are located.
The wavelength locker also must be insensitive to changes in the input optical power input to FP etalon. For example, a common strategy for frequency locking is to use the side of an etalon transmission peak for frequency discrimination. In this method the output signal from the etalon is compared to a fixed reference value and provides a measure of the deviation of the laser from the lockpoint. However, power fluctuations in the input light are also capable of producing changes in the etalon transmission signal, mimicking a frequency change and unintentionally detuning the laser from the desired lockpoint. To differentiate between a frequency change and a power change of the laser light, a power reference is measured that is independent of the etalon signal. The power reference signal is used to normalize the etalon transmission signal to render it insensitive to changes in the input optical power. In addition, the power reference signal may be used to monitor and control the power of the laser.
An alternative method to lock the laser wavelength to a reference interferometric optical element is a so-called dither lock (see FIG. 2). The goal of a dither lock is to quantify how the etalon responds to a time-varying wavelength, and then to use this information to control the time-averaged value of the laser wavelength (or, in even broader sense, some statistical moment of the wavelength distribution in time, of which the average is just one example). This is accomplished by comparing the known variations in the laser wavelength to the variations in the signal of the light that has interacted with the etalon. Thus the comparator receives two signals: one from the dither generator that represents how the wavelength is varying in time, and a second from the detector that represents the reaction of the light upon interacting with the etalon.
Using a mixer, phase sensitive detector, a multiplier, or other comparator, the response of the etalon to a laser beam with time-varying wavelength can be quantified. This xe2x80x9cwavelength responsivityxe2x80x9d is the output of the comparator. The circuit then compares the measured wavelength responsivity to a desired responsivity, and adjusts the laser to maintain a lock on the desired responsivity.
As an example, one typical wavelength responsivity of interest is the first derivative of the etalon response with respect to changes in wavelength. To measure the first derivative, a sinusoidal oscillator generates the wavelength dither. With proper phase adjustment, the output of the comparator is the first derivative of the etalon response. The first derivative is equal to zero at the peak of the etalon transmission function, which makes an excellent lock point for a dither lock system. With this type of sinusoidal dither and detection, the average value of the laser output wavelength is locked to the point of maximum transmission of the etalon.
Because a wavelength locker is a calibrated reference, its design must be insensitive to changes in the operating environment. The primary environmental concerns are temperature changes and polarization sensitivity of the beam splitters. Temperature changes to the etalon cause changes in the optical path length of the interferometric optical element (due to the material""s thermal coefficient of expansion and/or temperature-dependent refractive index), thereby changing the FSR and peak locations of the FP etalon and causing the laser to detune from the desired lock frequency. Thermally-induced changes to the etalon may be mitigated by constructing the etalon from temperature-insensitive materials, applying direct temperature control to the wavelength locker, or both.
Polarization sensitivity of the beamsplitters in the wavelength locker will change the power normalization signal, which shifts the lock signal and causes the laser to detune in frequency. There are several strategies to minimize polarization sensitivity. One approach is to use the free-space propagating beam in a laser module such that the state of polarization (SOP) is known. Thus, the coatings on optics and beamsplitters can be optimized for the SOP. Nonetheless, due to manufacturing tolerances and variable operating conditions, some mismatch between coatings and SOP may persist. A second approach is to make the coatings polarization dependent, but identical for both the etalon and power reference optical paths. An example is an identical coating to the reflective surfaces in a monolithically fabricated dual beam splitter. In this case, any shift in the polarization state results in the same change in reflectivity at both interfaces, and the reflectivity changes are common mode to the two measurement paths. The monolithic fabrication of a dual beamsplitter also ensures the angle of incidence is identical for each beamsplitter, which further minimizes differences in reflectivity due to the SOP. Finally, a third approach to polarization sensitivity is to choose coatings of the beam splitter that are altogether non-polarizing.
In addition to the basic technical requirements of a wavelength locker, the optical networking market requires that wavelength lockers possess a small size, exhibit a long lifetime and are inexpensive. Physical size constraints are strict because most communication lasers fit into industry-standard 14-pin xe2x80x9cbutterflyxe2x80x9d packages and many laser vendors require that the wavelength locker be internal to their laser package. Typical volume constraints for a wavelength locker are on the order of 30 mm3. In order to fit in such a confined space tight manufacturing tolerances and monolithic architectures are necessary.
The long lifetimes of telecommunications systems demand that wavelength lockers operate robustly over a very long periods ( greater than 20 years). Wavelength lockers must be constructed to avoid the outgassing of superfluous material, to survive mechanical and thermal shock, and otherwise not to age in any observable or detrimental way. Epoxies and adhesives in the beam path are notorious for age-dependent power losses, outgassing, and inadvertent etalon effects and must be eliminated from a design.
Finally, the large number of lasers needed for a communication system requires that the wavelength locker for each laser is inexpensive to manufacture in high volumes.
There is a need for a wavelength locker that has a small physical size. There is also a need for a wavelength locker with a size suitable for placement inside an associated laser package. There is a further need for a wavelength locker that operates robustly over very long lifetimes. Yet another need is for a wavelength locker that does not use epoxies and adhesives in the beam paths.
Accordingly, an object of the present invention is to provide a wavelength locker that has a small physical size.
Another embodiment of the present invention is to provide a wavelength locker with a size that is suitable for placement inside to an associated laser package.
A further object of the present invention is to provide a wavelength locker that operates robustly over very long lifetimes.
Yet another object of the present invention is to provide a wavelength locker that does not use epoxies and adhesives in beam paths.
A further object of the present invention is to provide a power reference signal for monitoring optical power and using said signal for the control of the output optical power of the laser.
Another object of the present invention is to provide a wavelength locker that improves the information-carrying capacity of an optical networking system.
A further object of the present invention is to provide a wavelength-locking apparatus that is highly configurable, easily manufactured, low-cost and very robust.
These and other objects of the present invention are achieved in a wavelength locker that includes a first beam splitter positioned in a beam path of an output beam produced by a laser. The first beam splitter splits the output beam into a first beam and a second beam. An interferometric optical element is optically contacted to the first beam splitter. The interferometric optical element receives the second beam from the first beam splitter and generates a third beam with an optical power that varies periodically with wavelength. A second beam splitter is positioned in a beam path of the first beam and creates a fourth beam. A first detector generates a first signal in proportion to an optical power of the third beam. A second detector generates a second signal in proportion to an optical power of the fourth beam. A wavelength of the output beam is adjusted in response to a comparison of the first and second signals and a predetermined reference signal level.
In another embodiment of the present invention, a wavelength locker has a dither generator coupled to and interacting with a laser to produce an output beam with a wavelength that varies in time. The dither generator produces a first signal that is representative of variation of the wavelength in time. A beam splitter is positioned in a beam path of the output beam and splits the output beam into a first beam and a second beam. A interferometric optical element is optically contacted to the beam splitter. The interferometric optical element interacts with the second beam to produce a third beam. A first detector is positioned to receive the third beam and produce a second signal that represents the reaction of the second beam with the interferometric optical element. A comparator is coupled to the first detector and the dither generator. The comparator is configured to receive the first signal at a first input, the second signal at the second input and produce an output that is proportional to a product of the first and second signals. The comparator compares the output to a reference and generates an error signal that is applied to the laser in order to drive a wavelength error to zero.